Update global navigation for now split Gitaly monitoring content

What does this MR do and why?

For: gitlab-org/gitlab#555179 (closed), let's update the navigation because of a content split happening in: gitlab-org/gitlab!197573 (merged).

• Before: https://docs.gitlab.com/administration/gitaly/
• Before: https://docs.gitlab.com/administration/gitaly/praefect/
• After: https://docs.gitlab.com/review-mr-972/administration/gitaly/
• After: https://docs.gitlab.com/review-mr-972/administration/gitaly/praefect/

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.